Dermatology Review

Stephen White, DVM, Dipl. ACVD, a professor in the Department of Medicine and Epidemiology at the University of California, Davis, School of Veterinary Medicine, and Anthony Yu, DVM, MS, Dipl. ACVD, associate professor of dermatology at the University of Guelph's Ontario Veterinary College, presented a whirlwind tour of just about every skin disease known in the horse. They described systematic diagnostic testing and treatment along each stop to attendees of the 52nd annual American Association of Equine Practitioners convention, held Dec. 2-6, 2006, in San Antonio, Texas. Skin ailments were broken down into classifications of pruritic (itching), nodular, or crusting lesions. The variety of problems discussed included common equine skin diseases like sarcoids, insect hypersensitivity, hives, allergies, photosensitivity, skin cancer, and a variety of fungal and bacterial infections.

Pertaining to the topic of the pruritic horse, one bacterial infection they discussed is currently of great concern due to emerging implications in public human health--Staphylococcus species. A greater prevalence of methicillin-resistant S. aureus (MRSA) poses a potential threat to humans that have close contact with infected horses. MRSA has been identified as colonizing "normal" horses and humans, yet can be devastating if clinical infection occurs.

When skin appears to have a variable scaling and non-scaling pattern, it was suggested that a fair presumptive diagnosis could be a bacterial infection. Rain scald (caused by Dermatophilus bacteria), with its typical paintbrush-appearing lesions, is a common bacterial skin infection in humid climates. In the presence of environmental moisture, areas of skin trauma are susceptible to infection from bacteria-contaminated crusts dropped from an infected carrier horse.

Other equine skin diseases that might initially create itching include fungal infections (dermatophytosis) such as those organisms that cause ringworm. Previous treatment strategies have used tamed iodine shampoos, but the experts suggested that chlorhexidine (disinfectant and topical anti-infective agent) shampoo products are more effective. One type of yeast, Malassezia spp., is known to cause rubbing of the tail and abdomen in mares infected between the mammary glands.

The most common itching problem in horses is related to insect hypersensitivity, particularly to Culicoides gnats. Horses react to the proteins in the gnats' saliva, the bites themselves, or to the inhaled desiccated (dried) insects. Other flies can cause bites that itch, but these Culicoides gnats seem to incite the most profound itching response, with horses mutilating their tails, manes, and torsos in attempts to relieve their misery. Barns and fences also suffer from their rubbing efforts. Culicoides sensitivity problems are typically seasonal, and certain breeds (like Icelandic ponies) seem to be predisposed. Stabling near standing water increases the risk. It is best to house sensitive horses at the top of a hill, away from wet, low-lying areas where these insects breed. There is also some speculation that vaccines might stimulate the ever-vigilant immune system and create a hyperreactive response to many allergens like mold spores, insect bites, etc.

Other allergic responses abound in equine skin. One example is atopy (a genetic predisposition toward the development of immediate hypersensitivity reactions) that causes chronic recurrent urticaria (hives) related to allergens from food or environmental or respiratory contaminants. Intradermal allergy testing is useful to identify offending proteins. The best management approach relies on keeping a horse's "allergic threshold" below a trigger point; this is accomplished effectively by controlling the environment through modifying stabling as necessary, implementing insect control, minimizing dust in bedding and hay, and keeping the diet simple.

The next topic along the tour focused on nodular skin diseases. These have multiple causes, ranging from bacterial infections such as sporotrichosis (a chronic fungal infection caused by Sporothix schencki and pigeon fever abscesses, to ulcerated or nodular summer sores from Habronema fly larvae (which develop in housefly and stable fly maggots living in manure and become infective when the fly larvae pupate and are carried by adult flies onto horses' open wounds), to neoplasia such as melanoma, mast cell tumors, or sarcoids. Sarcoids are reported in 35-90% of skin neoplasms, and although these are non-malignant, they are a cosmetic nuisance and can potentially interfere with tack and equipment. Sarcoids are commonly found in thin-skinned areas, especially in locations of previous wounds or continual contact irritation by tack. Different parts of the world have varying distributions of sarcoid lesions. For example, horses in warmer climates tend to have more sarcoids on the legs, while horses in colder climes tend to develop more lesions on the trunk. There is speculation that this might be due to insect feeding patterns related to different distributions of insects around the world.

Other nodular-type skin lesions include plaques in the ears caused by sarcoids or warts (papillomavirus), and nodular necrobiosis (tissue death) on the back and trunk caused by an allergic response to insect bites or saddle pad materials. Hives were discussed as nodular lesions, although the presence of hives is a symptom of an allergy and not necessarily a diagnosis.

Danny Scott, DVM, Dipl. ACVD, professor of medicine in Cornell University's College of Veterinary Medicine, said, "A lump is just a lump, and a bump just a bump, until it is biopsied."

To achieve an accurate diagnosis in many cases of skin disease, it is necessary to take a biopsy sample of the lesion and subject it to histopathologic examination.

The third group of skin diseases covered in-depth included crusting and ulcerated lesions, such as sarcoidosis (inflammation that causes tiny lumps of cells in various body organs) caused by an abnormal reaction to an infectious or non-infectious antigen. Another skin problem that creates crusting lesions arises from an autoimmune response known as pemphigus.

In the final section of the dermatology tour, hereditary skin diseases were discussed, including HERDA (hereditary equine regional dermal asthenia, formerly called hyperelastosis cutis, which causes affected horses' skin to be fragile and tear very easily) in horses with specific Quarter Horse lineage, and chronic lymphedema (lymphatic fluid buildup in soft tissues of the body) in draft horses.

Get research and health news from the American Association of Equine Practitioners 2006 Convention in The Horse'sAAEP 2006 Wrap-Up sponsored by OCD Equine. Files are available as free PDF downloads.

About the Author

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado,
and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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